Electro-discharge machining of metals and the like



tates This invention relates to the electro-discharge machining ofmetals for reproducing from or on a block of metal the form or surfacecontour of a model by electrically cutting or eroding the block of metalwith an electric spark or are from the model and, more particularly, toliquid hydrocarbon compositions in which such electric machining iscarried out with the model and the block of metal immersed in thehydrocarbon composition for enhancing the speed and efficiency of theoperation. This application is a continuation-in-part of copeudingapplication Serial -No. 13,411, filed October 22, 1959.

Techniques or processes of the character to which this invention relateshave to do with the well known procedure of electro-discharge machiningor (as hereinafter sometimes referred to) electro-erosion, whereby thereis obtained in a block of metal a reproduction of the desired surfacecontour or formation of a model of the desired finished article byelectrically eroding or cutting away the metal of the block of rawmaterial with a spark or are engendered by the model as an electrode.Thus, generally, such techniques involve the producing of an electricaldischarge between a model and the block or unfinished piece of metalonto which it is desired to impress or erode a reproduction (including anegative reproduction) of the configuration of the model. Such erosionor cutting is provided by having the model as, for example, the cathodeof an electrical circuit and the block of metal to be formed as theanode of the circuit.

With such an arrangement, as is well understood in this art, electricdischarge between the model-cathode and the rough block-anode erodes orcuts away those portions of the block-anode to correspond with orreproduce the surface of the model, and, particularly, as the model isadvanced toward the block of metal being worked during the operation ofthe process. That is, the impressed electric current provides that theblock of metal or other material being worked is constantly eroded, asthe anode, while the model or cathode is moved toward the metal beingworked, and, consequently, progressively is introduced toward or intothe metal being worked to tiorm a reproduction of the model in thesurface of the rough block.

Such metal working techniques are well known and, as will be understood,provide a method for rapidly and readily forming in a block of metalvarious shapes and configurations which would be quite difiicult toobtain by other more classic means of metal working such as turning,grinding, cutting, etc.

Such an electro-erosion forming operation may be carried out, as is wellunderstood, with the anode and cathode surrounded by air or variousother media. Certain enhanced results are achieved if the mediumsurrounding the anode and cathode at least in the portions thereof wherethe eroding or forming action occurs, is a liquid medium or a mediumother than air, advantageously circulated around the electrodes. Such aliquid medium has been found to produce a number of advantages inoperation. For example, by ionization when electrons initially areemitted by the cathode, it liberates suppleice mentary electrons whichgive up kinetic energy at the point of impact on the anode or blockbeing formed; similarly, a surrounding liquid medium absorbs heat energyliberated during the operation and also may aid materially in thephysical elimination of particles of metal eroded by the electricaloperation from the anode.

As noted above and as is well understood in this art, it is particularlydesirable, from the standpoint of enhancing the economies of acommercial operation involving electro-erosion, to increase the speedwith which the eleotr-o-eroded reproduction can be achieved (i.e., thespeed with which satisfactory results are obtained in advancing thecathodic model toward the anodic metal to be formed), as well as byreducing as much as practicable the erosion or consumption of the modelelectrode in use, and, just as some enhancing of these desirablequalities may be achieved by utilizing a liquid medium in which theelectrodes are immersed during the electroerosion operation, so also maysuch advantages of a liquid bath be further enhanced by the particularchoice of composition of the liquid bath.

According to this invention, then, a hydrocarbon liquid medium isprovided, particularly adapted for use in an electro-erosion operationof the character described, and utilizing a dielectric hydrocarbonliquid which, particularly, permits augmenting the speed of erosion andpermits, for the same consumption of energy, an increase in the yield ofthe process as well as substantially diminishing the consumption orerosion or necessary wastage of the cathodic driving model or electrode.Such liquid medium comprises primarily hydrocarbons having acarbonto-hydrogen weight ratio of equal to or greater than 7, whichhydrocarbons are readily obtained from various treatments of certainpetroleum fractions and otherwise in accordance with this invention asnoted in more detail below; and, preferably, such liquid hydrocarbonmedium includes aromatic and solvent-extracted cuts or fractions ofpetroleum distillates and/or monocyclic monoalkyland polyalkyl-benzenesof the general formula C H and having the above noted carbonhydrogenweight ratio, all treated in accordance herewith to produce a liquiddielectric medium adapted for achieving the enhanced results disclosedin an electroerosion operation of the character to which this inventionrelates.

With the foregoing and other objects in view, as will be apparent fromthe disclosure herein and from the following description and theappended claims, several illustrative embodiments of materials and stepsembodying and for practicing this invention may now be described insomewhat greater detail.

As has been noted, there are certain advantages to be obtained, with anelectro-erosio-n operation of the character to which this inventionrelates, if the electrodes are surrounded by a liquid dielectric medium,instead of air or other gaseous media. Among these, as noted, may becounted the enhancement of an ionizable liquid media, the aid incontrolling and dissipating heat engendered at the eroding surface bythe electric discharge, and the fluid dissipation of metallic particlesremoved from the surface of the rough block or workpiece being eroded orformed into the desired final configuration. It has been found, however,and particularly in accordance with this invention, that certainimportant enhanced advantages are obtained in connection with theutilization of liquid compositions embodying this invention in suchelectro-erosion operations, and that such advantages are substantiallyunexpectedly enhanced overrthe results achieved with other liquidpreparations previously utilized in such operations, and particularly bycomparison with similar hydrocarbon substances having acarbon-to-hydrogen weight ratio of less than 7, at which a point adiscernibly sharp diiference in desirable characteristics occurs(although, perhaps, for reasons which may not now be completelyunderstood) by comparison even with hydrocarbons having less than suchratio but incidentally other characteristics (such as viscosity, boilingpoint, flash point, etc.) which might suggest operative or desiredapplicability thereof for the purposes in accordance herewith.

Thus, particularly satisfactory results have been obtained in accordanceherewith by using, as the liquid immersion medium for electro-crosionprocess of the character described, aromatic hydrocarbons having acarbon-hydrogen ratio in excess of 7, which hydrocarbons are extracted,in known manner, from various petroleum distillates and treated, inaccordance herewith, to produce the particularly adapted liquid mediumfor achieving the enhanced results hereof. Similarly, satisfactory orpreferred results have been achieved in accordance herewith by using,particularly, aromatic hydrocarbons belonging to the family ofmonocyclic monoalkyl and/ or polyalkyl benzenes, as noted above, withthe desired carbon-hydrogen weight ratio. Other desirablecharacteristics of hydrocarbons for use in connection herewith will beunderstood and are well known in the art of electrodischarge machining(and, particularly, for so-called inside machining where the entireworkpiece is immersed in the dielectric bath, as compared with outsidemachining where merely a film of dielectric liquid is maintained betweenthe electrode and a workpiece forming a spark gap therewith).

Also, as will be understood, some of the characteristics of thehydrocarbon liquids in accordance herewith and/or the dielectric bathsincluding such hydrocarbons may vary somewhat depending upon or inaccordance with the particular machining operation being used and/ orsuch characteristics thereof as the particular type of apparatus beingused, the desired speed of cutting, the configuration being cut, etc.For example, in most instances one would not desire to utilize ahydrocarbon having an excessively high vapor tension such as associatedwith hydrocarbons boiling above about 260 C. at atmospheric pressure.Similarly, as will be understood, hydrocarbon baths in accordanceherewith may comprise one or another of individual hydrocarbonsdisclosed, or mixtures thereof, or mixtures of various hydrocarbonswhich, individually and alone, may not come within the definitionshereof provided that such mixture thereof in the bath as used is inaccordance with the ranges of weight ratios of carbon to hydrogen, etc.,in accordance herewith.

Also, as will be understood, such hydrocarbon materials may be obtainedfrom a variety of sources and by a variety of diiferent means orprocesses. For example, they may be produced by thermal or catalyticdecomposition or conversion of hydrocarbons, by chemical synthesistechniques, by physical separation or distilla tion, or by combinationsthereof. Satisfactory results in accordance herewith have been achievedwith hydrocarbon materials prepared, in known manner, by such well knowncracking processes as catalytic platforming, or reforming orhydrofining, by steam cracking, etc., whereby cuts or fractions areproduced rich in alkylsubstituted aromatic hydrocarbons. Similarly,various distillation separation or fractionating techniques have beenused, in known manner, as well as solvent extractions such as with furfural, phenol, sulfurous anhydrides, nitrobenzene, dimethyl-formamideand its analogues, etc. It may be particularly noted that, in moderncommercial or industrial practice, various of the alkylbenzenes are alsobeing produced more and more by the direct alkylation of various simpleraromatic hydrocarbons, such, for example, as dodecy-lbenzene byalkylation of benzene with :tetramers of propylene, with the productionof, in addition to the desired product, a non-aromatic fraction andfractions alkylated to various degrees such as etc.-, benzenes, whichmaterials, suitably selected in ac cordance herewith satisfactorilyprovide hydrocarbon liquids for electro-discharge machining, and havebeen utilized by themselves or admixed with other materials orconcentrated by solvent extraction and/or distillation, as noted above.

As purely illustrative of materials and treatments in accordanceherewith and for achieving the enhanced results hereof inelectro-erosion processes regarding the above noted increase in speed ofthe cutting or eroding or forming cycle, the reduced consumption of themodel electrode, etc., may be noted several particularly satisfactoryexamples, although these following examples are set forth, as will beunderstood, merely as further illustrative of the invention and not by:way of limitation thereof.

For example, satisfactory results according to this invention wereachieved by taking a fraction of Middle East crude petroleum having beendistilled and separated to provide a cut of "a viscosity (at 210 F.) of3.9 centistokes, a density (at 115 C.) of 0.894, and a refractive index(11 of 1.4842. Such distillate was initially dewaxed to a pour point of-12 C., and then was submitted to a fur-fural extraction using 180volumes of furfural for each volumes of the petroleum cut.

According to such treatment, there was obtained, after a separation ofthe furfunal phase and after a stripping or distillation of the solvent,a residual material for use in accordance with this invention having thecharacteristics, among others, as follows:

PRODUCT A Density at 15 C 1.031 Refractive index 11 1.5751 Viscosity(centistokes):

at 100 F 9 2.19

at 50 C 44.0

at 210 F 6.77 Flash point (L-uchaire), C Four point, C -12 Weight ratioof C/H 9.2

As noted, satisfactory results were obtained with the foregoingpreparation in an electro-erosion machine of known construction, withthe foregoing preparation as an immersion fluid or dielectric medium,instead of other petroleum or lubricating oils heretofore utilized, toproduce the enhanced results noted in the electro-erosion or forming ofa metal article from hard steel.

As illustrated of the enhanced advantages achieved by utilizingcompositions embodying and for practicing this invention, as comparedwith conventional lubricating oil media, for an electro-erosionoperation of the character described, one may note centain comparisonsre.- garding such considerations as the impressed potential, the speedof forming the desired article in the electroerosion operation, and theamount or rate of consumption of the model electrode or forming anodewith different liquid baths. For example, comparisons have been made asbetween the Product A noted above and either or both of a moreconventional liquid bath for an electroerosion operation composed of,for example, materials resenting the following characteristics:

Product Product B 0 Density at 15 C 0.ss7 0.874 Refractive Index 1.463 1. 465 Viscosity (Gentistokes):

At 100 F 20.2 40, 6 At 50 0.-.. 14

3. 9 6.1 l2 l2 6. 4 G. 3

Merely for comparing and emphasizing the enhanced advantages of acomposition such as Product A in accordance with this invention withcompositions such as Product B or Product C as previously used as liquidimmersion media for an electro-erosion process of the character to whichthis invention relates, one may note that, in electro-erosion apparatusas above noted, comparative data were obtained in connection withforming a block of extra-hard steel (with a carbon content of about 1%)by measuring, among other things, the volumes of metals eroded orremoved from the steel block (as the anode) per unit time for comparableor equivalent operations involving separately the several liquid mediamentioned, such measurements forming, as will be understood, anexpression of the speed of cutting or erosion in the forming operation,and such data were obtained as a function of the electric potentialimpressed upon the system. Thees data are tabulated in the followingTable I:

Table 1 Product A Product B Product As will be understood from theforegoing, the speed of cutting or erosion is, quite desirably,approximating 40% to 50% higher with the same impressed electricalpotential when the surrounding liquid medium is one in accordance withthe provisions of this invention, as compared with the previously knownProducts B and C. As will also be understood, however, the speed ofcutting or erosion is not, in a commercial or mass-producing operation,necessarily the only element of important consideration.

Thus, for optimum efficiency in an electro-erosion operation of thecharacter to which this invention relates, one should consider theconsumption or rate of consumption of the cathodic electrode model,because, indeed, the overall efficiency may not, necessarily, beenhanced if the speed of cutting is raised or increased only with asacrifice in the original model, as by a concomitantly increased rate ofconsumption thereof requiring, of course, eventual or frequentreplacement of the model.

Utilizing compositions embodying and for practicing this invention,however, it is noted that, for any given electrical potential, theproducts embodying this invention, while producing a faster forming orcutting or eroding rate, also achieve a status of operation where theanodic model is less consumed.

Comparative data illustrating this enhanced advantage according to thisinvention may be noted as follows:

Table 11 Electric Potential (kw.)

Consumption of Cathode: 100x mm loss at cathode mm eliminated at anodeProduct A- Product B Product 0.

mam

6 erosion process according to this invention along both the lines ofincreased rate of cutting and decreased rate of cathode consumption.

As further illustrative of the enhanced advantages achievable withcompositions embodying and for practicing this invention as oilimmersents for the cutting electrodes of an electro-erosion device ofthe character described and as mentioned above, one may note a number ofcomparative data collected with regard to an other composition inaccordance with this invention, which will be here designated as ProductD.

Thus a distillation cut of crude petroleum having a viscosity ofapproximately 7.2 centistokes at 200 F. was de-waxed'and treated with anextracting treatment of furfural. The portion of this cut soluble infurfural Was separated by solvent extract and by distillation. Tovolumes of this separated product was added some 20 volumes of keroseneto adjust the viscosity thereof to a desirable range, and the resultingadmixture (as Product D) exhibited the following characteristics:

PRODUCT D Density at 15 C. 0.965 Viscosity (centistokes):

at F. 34.9

at 200 F. 4.6 Flash point (Luchaire), C. 55 Pour point, C. 9 Weightratio of C/H 8.10

As noted, with other compositions embodying and for practicing thisinvention, an enhanced comparative result is shown over otherlubricating oil immersion media for electro-erosion systems. Forexample, comparative data are noted below in Table III comparing theabove noted Product D with the previously noted Product B as follows(the units of notation being the same as above):

As a further comparison of the enhanced results attributable tocompositions embodying and for practicing this invention, one may notethe comparative results achieved with, for example, a lubricating oilcomposition such as that described above as Product C, as compared withanother product according to this invention. For example, satisfactoryresults are obtained with a composition (which may be herein designatedas Prodnot B) being produced from a dc-waxed petroleum crude submittedto an extraction treatment by a substance such as phenol, with theextracted products in the phenol phase being mixed with a 10%hydrocarbon extract from a cut of kerosene washed with liquid sulfuricanhydride. The designated mixture (Product E) has the characteristicsnoted below:

. PRODUCT E Density at 15 C. 0.988 Viscosity (centistokes):

at 100 F. 73.4 at 210 F. 6.4 Pour point, "C 20 Weight ratio of C/H 8.48

When comparative runs were made using and comparing the results of theabove mentioned Product C, for example, with the above recited ProductB, it was found that the Product E in accordance with this inventionproduces (in accordance with the units of notation previously mentioned)a speed of cutting or formation (at a 6 kw. potential of approximately685 as com- 6 pared with the aforementioned speed of cutting or ProdnotC of 515 (both calculated in terms of mm. per min. as above noted).Furthermore, the consumptions of the model or cathodic electrode,expressed as shown in Table II, are 15 with the Product C, andonly 11with the Product E of the present invention.

One interesting property of the hydrocarbon compositions embodying andfor practicing this invention should be emphasized, as will be notedfrom the foregoing, in that the utilization of the various compositionsherein in an electro-erosion operation or technique of the characterdescribed as dielectric media for immersion of the various electrodesproduces a certain superficial hardening, perhaps by cementation on theparticular part or by tempering the particular metal to the electricheating thereof and the bathing thereof in the liquid medium disclosed,so that a definite metallurgical advantage is obtained. Actually, suchenhanced advantages may include the aspect of the superficial hardeningto a certain extent through the surface of, for example, steel treatedin accordance with the foregoing, and the tempering thereof, as onewould expect from the intense heating by the electrode discharge ofrough steel in a bath of oil, as one might temper a steel member byheating it and then quenching it in a bath of oil. It is interesting tonote the comparative data which, according to the foregoing, has beendeveloped from various satisfactory uses of, for example, Product A ascompared with Product B in an operation embodying and in accord ancewith this invention. For example, comparative data in Table IV below,illustrative of the invention, is set forth as between a Product A inaccordance with the invention and the previous Product B:

Naturally, the metallurgical qualities of any material treated inaccordance with this invention may be expected to be altered as thematerial is heated, in connection with the electro-erosion thereof, inthe presence of a circulating liquid in accordance with this invention,and, as will be well understood, the superior treatment of any materialin accordance with this invention will, as is well understood, becalculated so as to accommodate whatever treating or other metallurgicalor crystalographic changes may be undergone in accordance with theelectro-erosion heating as immersed in one or another of the materialsor compositions set forth herein.

As will be understood, of course, the electro-erosion treatmentaccording to this invention, dependingupon, to some extent, theparticular composition of the metal being treated, may, indeed, have noeflect whatsoever, metallurgically, on the metal being treated.Actually, as will be noted from the foregoing and as well understood inthis art, the surface hardening mentioned above depends, on a largemeasure, on the viscosity, among other factors, of the composition inwhich the metal being treated is immersed during the electro-erosiontreatment. For example, it is quite possible, by the appropriateadmixture of various substances in the immersing liquid, to regulate toa substantial extent the amount of temper or superficial hardness of thefinal workpiece. As illustrative of the foregoing, one may note thefollowing example, which is to be comprehended within this invention,but is merely set forth herein as illustrating the tempering or surfacehardening effect (or the avoidance thereof) achievable as a concomitantor auxiliary efiec-t of the electro-erosion process in accordance withthe instant in- ,vention.

For example, taking Product A as previously described and fractionatingthis particular hydrocarbon substance by distillation in a vacuum, it ispossible to recover a head fraction containing approximately 25% of thetotal product previously described-which head fraction is to bedesignated here as Product F.

As illustrative of such an operation, a number of comparative data wereobtained with regard to these various Products A and F, and mixturesthereof. As further illustrative, one might note that an equal mixtureof such Product A and Product P (which mixture should be considered hereas Product G) was utilized in applying this invention to a block ofextra-hard steel, with satisfactory results, and with the comparativeresults, all indicating the enhanced advantages attributable to liquidmedia embodying and for practicing this invention as, for example, areset forth in Table V:

As additionally illustrative of other materials with which satisfactoryresults have been achieved in accord ance herewith may also be noted thefollowing:

PRODUCT H An alkyl-substituted mono-aromatic hydrocarbon correspondingapproximately to the formula 0 1-1 boiling between about 205 225 C. atatmospheric pressure, and having a carbon-hydrogen weight ratio of about8.05 was used as the dielectric liquid bath for an electro ero'sionmachining operation on a block of extra hard steel (at about 1% carbon)with a copper electrode under power input of the order of 1 ,kw. at afrequency of about 3600 hertz, to achieve satisfactory cuttingconditions and a desirably fine surface finish. By comparison withcomparable results obtained in working under the same conditions butusing as the dielectric bath an ordinary light petroleum or kerosenefraction usually employed in such an operation, the cutting speed herewas increased by a factor of about 1.65 that of the conventionaltechnique, while the wastage or consumption of the copper electrode wasonly about half that experienced with the conventional hydrocarbonoutside the ranges in accordance herewith.

PRODUCT I accordance herewith using hydrocarbons near the minimumrecited ratios, comparisons of the results achieved are noted below inTable VI, in which the cutting speed is noted in mmfi/ minute and theelectrode consumption in terms of the percentage ratio of volume ofmaterial lost at the cathode to volume eliminated at the anode:

Table VI Speed Consump tion Dodecylbenzene 288 a 16 Deodorlzed Kerosene31 '9 Considering the results obtained with deodorized kerosene as abase index of 100, the foregoing indicates that the utilization of thedodecylbenzene (with a carbon-hydrogen weight ratio only just above theminimum of 7 disclosed herein) gives a cutting speed index of 150, butan electrode-consumption index of only 52.

PRODUCT J A dielectric bath in accordance herewith was formed from aresidue fraction of the distillate from a catalytic reforming processhaving the following characteristics:

Such hydrocarbon product was utilized in an electr c-discharge machiningoperation as noted withthe two previous examples, and satisfactoryresults were achieved with a cutting speed of 265 mm. /minute, and Withan electrode consumption of only 19%. By comparison with the above noteddeodorized kerosene and results as a base index of 100, Product Iproduced the results of a speed index of 140 and anelectrode-consumption index of only 61.

PRODUCT K A light fraction of alkylated products provided from themanufacture of dodecylbenzene by alkylation of henzene with tetramers ofpropylene was utilized and evi denced the following characteristics:

Density at 15 C 0.862 A.S.T.M. distillation, C.:

Initial 94 166 20% 187.5 50% 205 80% 276 90% 35 6 Final 370 Aromaticcontent (wt. percent) 100 Weight ratio of C/H 7.9

In the utilization of this product K as above, a cutting speed of 260mm. minute was achieved with an electrode consumption of only 19%,which, by comparison with the above noted index of deodorized kerosene,results in a speed index of 135 but a consumption index of only 61. Bycontrast, and as further exemplifying the sharp or pronounced change indesirable characteristics of hydrocarbons at the minimum weight ratiovalue of 7 in accordance herewith, a heavier alkylated fraction providedby the same manufacture of dodecylbenzene, but having a density of 0.876and distilling between 325 C. and 380 C., and a carbon-hydrogen weightratio of only 6.95 produced results appreciably and significantlyinferior in the same electro-discharge machining operation, and notgreatly or significantly better than a kerosene bath.

Whereas, as will be understood, this invention is not limited to theutilization of the particular materials set forth in the variousillustrative examples above, it may be noted that the alkyl benzenematerials within the weight ratio disclosed have produced particularlyfavorable results especially when it is desired to obtain a fine finishon the surfaces being machined, as well as when it is particularlydesired to utilize relatively low power input (of the order of 1 kw.)for the machining operation. If, on the other hand, it is desired toutilize a higher power input for machining, other aromatic extracts maybe preferred, such as those obtained in the course of solvent refiningof petroleum distillates during lubricating oil manufacture, asexemplified by such materials as noted above as Product A or Product B,etc. In such cases, also, it may be desired to adjust the viscosity ofsuch heavier materials to a more convenient range by an admixture ofdiluent for the dielectric bath since, as will be understood, suchextracts from processes of manufacturing lubricating oils normallyproduce viscosities in a more or less high range.

According to the foregoing, as will be noted, a definite increase in theefiiciency and/or other optimum utilization of electro-erosion processesof the character to which this invention relates results fromutilization, as an immersion medium for the opposed electrodes of such aprocess, the compositions embodying and for practicing this invention,and, also, as noted in the foregoing, such liquid compositions are,indeed, available from the various hydrocarbon and/or petroleumfractions disclosed. Accordingly, in such eleotro-erosion operations, itnow becomes possible, in connection with the utilization of compositionsembodying and for practicing this invention, to achieve a degree ofspeed and/ or eficiency substantially beyond that which may have beenobtainable by the previously utilized liquid media noted.

While this invention has been described and exemplified in terms of itspreferred embodiments, those skilled in the art will appreciate thatmodifications can be made without departing from the scope of theinvention as defined in the appended claims.

What is claimed is:

1. In a method of the character described for the electric dischargemachining of a block of metal by cutting the metal with an electricaldischarge from an electrode, the steps which comprise immersing saidblock of metal and said electrode in a dielectric bath comprisingaromatic petroleum hydrocarbon, said hydrocarbon bath having acarbon-to-hydrogen weight ratio in excess of 7, and maintaining saidmetal in said bath during said electric dis charge machining thereof.

2. In a method of the character described for the electric dischargemachining of a block of metal by cutting the metal with an electricaldischarge from an electrode, the steps which comprise immersing saidblock of metal and said electrode at least in the area of electricaldischarge therebetween in a dielectric bath comprising solventextracted, aromatic petroleum hydrocarbon, said hydrocarbon bath havinga carbon-tohydrogen Weight ratio in excess of 7, and maintaining saidmetal in said bath during said electric discharge machining thereof.

3. In a method of the character described for the electric dischargemachining of a block of metal and the like by cutting the metal with anelectrical discharge from an electrode model of the final configurationto which the block of metal is to be machined, the steps which compriseimmersing said block of metal and said electrode model in a dielectricbath consisting essentially of aromatic petroleum hydrocarbon, saidhydrocarbon bath having a carbon-to-hydrogen weight ratio in excess of7, and maintaining said metal in said bath during said electricdischarge machining thereof for tempering said metal in said bath andunder the influence of heat from said electric discharge.

4. In a method of the character described for the electric dischargemachining of a block of metal and the like by cutting the metal with anelectrical discharge from a cathode model of the final configuration towhich the block of metal is to be machined, the steps which compriseimmersing said block of metal and said cathode model at least in thearea of electrical discharge therebetween in a dielectric bathconsisting essentially of solvent extracted aromatic petroleumhydrocarbon, said hydrocarbon bath having a carbon-to-hydrogen weightratio in excess of 7, maintaining said metal in said bath during saidelectric discharge machining thereof for tempering said metal in saidbath and under the influence of heat from said electric discharge, andadjusting the viscosity of said hydrocarbon bath by admixture therewithof a difierent liquid hydrocarbon dielectric for controlling the extentof said tempering action, said carbon-to-hydrogen weight ratio of saidbath still being in excess of 7 after said admixture of said differenthydrocarbon.

"5. In a method of the character described for the electric dischargemachining of a block of metal and the like by cutting the metal with anelectrical discharge from an electrode, the steps which compriseimmersing said block of metal and said electrode in a dielectric bathcomprising a furfural-extracted aromatic petroleum hydrocarbon fraction,said hydrocarbon bath having a carbon-to-hydrogen weight ratio in excessof 7, maintaining said metal in said bath during said electric dischargemachining thereof, and adjusting the viscosity of said hydrocarbon bathby admixture therewith of a difiFerent liquid hydrocarbon dielectric forcontrolling the effect thereof on said metal during said machining, saidcarbon-tohydrogen weight ratio of said bath still being in excess of 7after said admixture of said different hydrocarbon.

6. In a method of the character described for the electric dischargemachining of a block of metal and the like by cutting the metal with anelectrical discharge from an electrode, the steps which compriseimmersing said block of metal and said electrode in a dielectric bathcomprising a phenol-extracted aromatic petroleum hydrocarbon fraction,said hydrocarbon bath having a carbonto-hydrogen weight ratio in excessof 7, maintaining said metal in said bath during said electric dischargemachining thereof, and adjusting the viscosity of said hydrocarbon bathby admixture therewith of a different liquid hydrocarbon dielectric forcontrolling the eflect thereof on said metal during said machining, saidcarbon-tohydrogen weight ratio of said bath still being in excess of 7after said admixture of said different hydrocarbon.

7. The method of electric discharge machining as recited in claim 1 inwhich said dielectric bath comprises a monocyclic alkylated benzenehydrocarbon having the general formula C H 8. The method of electricdischarge machining as re cited in claim 1 in which said dielectric bathcomprises dodecylbenzene.

9. The method of electric discharge machining as re- References Cited inthe file of this patent UNITED STATES PATENTS 2,628,330 Williams Feb.10, 1953 2,818,490 Dixon et al Dec. 31, 1957 2,884,313 Browne Apr. 28,1959 2,962,577 Webb Nov. 29, 1960

1. IN A METHOD OF THE CHARACTER DESCRIBED FOR THE ELECTRIC DISCHARGEMACHINING OF A BLAOCK OF METAL BY CUTTING THE METAL WITH AN ELECTRICALDISCHARGE FROM AN ELECTRODE, THE STEPS WHICH COMPRISE IMMERSING SAIDBLOCK OF METAL AND SAID ELECTRODE IN A DIELECTRIC BATH COMPRISINGAROMATIC PETROLEUM HYDROCARBON, SAID HYDROCARBON BATH HAVING ACARBON-TO-HYDROGEN WEIGHT RATIO IN EXCESS OF 7, AND MAINTAINING SAIDMETAL IN SAID BATH DURING SAID ELECTRIC DISCHARGE MACHINING THEREOF.